In order to improve the recovery of hydrocarbons from oil and gas wells, the subterranean formations surrounding such wells can be hydraulically fractured. Hydraulic fracturing is used to create small cracks in subsurface formations to allow oil or gas to move toward the well. Formations are fractured by introducing specially engineered fluids at high pressure and high flow rates into the formations through the wellbores.
The fracture fluids are preferably loaded with proppants, which are usually particles of hard material such as sand. The proppant collects inside the fracture to permanently “prop” open the new cracks or pores in the formation. The proppant creates a plane of high-permeability sand through which production fluids can flow to the wellbore. The fracturing fluids are preferably of high viscosity, and therefore capable of carrying effective volumes of proppant material.
Recently, there has been an effort to monitor hydraulic fracturing and produce maps that illustrate where the fractures occur and the extent of the fractures. Current hydraulic fracture monitoring comprises methods of processing seismic event locations by mapping seismic arrival times and polarization information into three-dimensional space through the use of modeled travel times and/or ray paths. Travel time look-up tables may be generated by modeling for a given velocity model. Non-linear event locations methods are commonly used to locate the seismic event location.
Non-linear event location methods involve the selection and time picking of discreet seismic arrivals for each of multiple seismic detectors and mapping to locate the source of seismic energy. However, to successfully and accurately locate the seismic event, the discrete time picks for each seismic detector need to correspond to the same arrival of either a “P” or “S” wave and be measuring an arrival originating from the same event. During a fracture operation, many hundreds of microseismic events may be generated in a short period of time. Thus, one difficulty is to correctly identify the source of each microseismic event and to correctly identify an arrival time with a correct wave-type (e.g., P-wave, S-wave). If a picked arrival time is badly incorrect or the picked arrival time is for the wrong wave type, the estimated location of the origin of the seismic event is likely to differ significantly from the true location.
Current techniques commonly employed in the industry require considerable human intervention to quality control the time picking results. It can often take weeks from the time of recording and detecting the microseismic events to produce accurate maps of the event locations. Even so, the result, which requires human interaction and interpretation, can lead to multiple and non-reproducible solutions.